The purpose of this investigation is to obtain information on the nature of the regulatory mechanisms that control erythropoiesis, through analysis of the action of mutant genes that alter the normal growth and development of blood-forming tissue. Twelve of the thirteen known hereditary anemias of mice are under study. They provide the material essential for a comparative, developmental, biochemical, and genetical analysis of hematopoiesis. The information acquired is developing a generalized picture of normal mammalian blood-formation, indicating the nature of the controls, the presence and function of alternative regulatory mechanisms, and the potentiality for therapy or permanent repair of derangements produced by genetic, chemical, or environmental agents. The ultimate aim of this investigation is to provide information and techniques for new approaches to, and improved techniques in, the therapy of human hereditary anemias.